I've been told I need to replace the rear trailing arm bushes on my 318is Coupe 1999 Plate. Pleaes can anyone advise on whether any other work needs to take place as a result of this, such as wheel tracking/allignment.

Also, please could anyone let me know how long this kind of job takes, if it is complex and how much they have been charged for the work if taken to a garage. I seem to be experiencing the rattling / knocking and am looking to get the work carried out this week to prevent further damage or related issues.

You'll need an alignment afterwards, unless you manage to reinstall the console/trailing arm in exactly the same orientation as before you took it off the body of the car. Check bimmerdiy.com, there are a couple of writeups there on trailing arm bushings, that should help you judge whether you can tackle the job yourself

I suggest you do it.
Search for posts by me about RTAB's
check unitedbimmer.com and also search there for posts by me about RTAB's.

The bushings hold the wheel's alignment, if they are worn, they allow the wheels to move OUT of alignment during driving, IE: when you corner hard, hit bumps, accelerate, and brake. RTAB's are very noticeable, and once you replace them, you'll immediately feel a much stiffer rear end.

Get Ground Control shims while you are at it to prevent further wear and limit the bushing's range of motion.

Search the forum about Lower Control Arm Bushings as well (AKA: LCAB or LCA) You might want to replace your tie-rods as well.

Mpact has the bushings and GC shims for 79$.
I strongly suggest you get the shims. Unless you have seen how far the RTA can (and *DO* when the bushings need replacing) move inside the console, you probably wont appreciate the effectiveness of the shims. The shims alone are 60$, the combo kit would save you 15$ had you bought them from bavauto. Plus, I have had bad experience with bavauto's shady parts. Their parts are usually aftermarket and better prices and deals can be found elsewhere.